Nitric acid is produced commercially by passing ammonia and air across a gauze woven from platinum-rhodium-palladium alloy wire. The ammonia, initially, is converted to nitric oxide over the precious metal gauze, and the nitric oxide is, subsequently, oxidized and absorbed to form nitric acid. The ammonia oxidation to nitric oxide generates a large exotherm and raises the temperature of the catalyst in the range of 810.degree. C. to 960.degree. C. During the ammonia oxidation process, some of the precious metal is volatilized from the surface of the gauze wire. The rate of loss is dependent on the temperature, pressure, and flow rate of gases across the catalyst surface. The cost of the precious metal lost from the ammonia oxidation catalyst is a significant part of the cost of operating a nitric acid plant.
The typical methods of recovery of the precious metals volatilized from the oxidation gauze are mechanical separation methods such as filtration, or platinum vapor capture by a palladium alloys system, the so-called "getter" system.
The prior art getter systems were comprised of flat layers of palladium or palladium alloy woven mesh separated by oxidation resistant base metal screens. In U.S. Pat. No. 4,497,657 the collection efficiency of the individual layers of the getter catchment system are maximized by utilizing high surface area to weight ratios by using high mesh to wire diameter products. A getter catchment system was introduced for commercial use, constructed of alternating layers of woven palladium alloy mesh and oxidation resistant base metal screens that were corrugated to a simple sinusoidal like configuration with a maximum developed curve to flat ratio of .pi./2:1. The increased developed curve to flat (C/F) ratio increased the surface area per cross sectional area of the reactor. The sinusoidal like corrugated pattern changes the developed curve to flat ratio in a single direction in the plane perpendicular to the gas flow. This is currently marketed commercially as a Low Pressure Drop (LPD), Mass Transfer Limited (MTL) getter catchment system.
The collection efficiency of the individual layers is a function of the net frontal area of the getter that is presented to the gas stream. This frontal area is controlled by the individual wire diameter and the meshes per inch, and the developed curve to flat (C/F) ratio. These system, typically, have recovery efficiencies of 30 to 80 percent depending on the operating conditions of the nitric acid plant and the design of the catchment system.
By curve to flat ratio is meant the ratio of that part of the wire that is not in the base plane of catalyst element (sheet gauze, etc.) to that part that is in the base plane of the element. For curved wire, it is the ratio of the curve above the base to that in the base plane ("flat") of the element.
A typical platinum recovery system for an ammonia oxidation catalyst gauze system and its use is shown and described in U.S. 4,412,859, U.S. 4,526,614 and U.S. 4,497,657, all of which are hereby incorporated by reference, in toto. These gauzes may be made by the method in U.S. Pat. No. 3,470,019, see Example 19, and other known methods.